Arkham and Jester were two different entities in one, Arkham being his human form, and Jester being his demonic form he'd gained. Vergil looks like Dante in his human form, and has a blue DT in his demonic form. So the theory that Vergil's appearance is deceiving is not plausible, because he just looks that way, he can't have three different faces, get what I'm saying?
And Nero is related to Vergil and Dante, because he is called a descendant of Sparda in the game by Sanctus, and Dante hints at it straight in the beginning with his 'looks like you too are a...'. Either way, Nero has Sparda's blood pumping through his veins, which makes him family, it doesn't matter whether he's some long lost second cousin or whatever. Sparda's blood binds them, makes them family, end of story.
How did this thread veer so far off track anyway? I wanted to avoid bringing Dante and Vergil in here, darn it!